The present invention relates to an improved method of transferring an electrostatic developed image from a photoreceptor to a transfer member in contact with the image. More particularly, the invention is directed to a method for charging the transfer member to a polarity opposite to that of the toner forming the developed image while simultaneously charging the photoreceptor to the same polarity as that of the toner particles.
In conventional xerography, a xerographic member comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic latent images. In the xerographic process, the photoconductive surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas where light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Where the charge is greater, a greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. Generally, the developed image is then transferred to a suitable transfer member (e.g., a sheet of paper), and the image is affixed thereto to form a permanent record of the original document.
In the practice of xerography, the transfer member is caused to move into synchronized contact with the developed image during the transfer operation, and an electrical potential opposite from the polarity of the toner is applied to the side of the transfer member remote from the photoconductive surface to electrostatically attract the toner image from the surface to the transfer member. With this conventional method, however, the transfer fields are generally not sufficient to overcome all of the forces at the toner-photoreceptor interface, thus leaving larger amounts of charged residual toner than desired on the photoconductive surface to be cleaned. Because larger amounts of toner are left on the photoreceptor than desired, slightly higher development is required to compensate for the untransferred toner, thus increasing the amount of toner needed to develop a given number of latent images. Consequently, what is needed is a more efficient method of transferring developed images, one which transfers a greater percentage of toner from the photoreceptor to a transfer member than do conventional methods.
Consequently, a primary object of the present invention is to provide a transfer method which is more efficient than conventional methods, a method which produces higher toner-photoreceptor transfer fields than conventional methods.